1. Field of the Invention
The present invention relates to a deposited film forming apparatus constructed to generate a plasma between a power applying electrode connected to a high frequency power source and a substrate disposed in opposition to the power applying electrode and serving as an electrode in a vacuum chamber to decompose a reactive gas introduced into the vacuum chamber to form a thin film as a deposited film on the substrate, and to a deposited film forming method using the deposited film forming apparatus.
2. Related Background Art
In the conventional deposited film forming apparatus, there are a flat plate type base member grounded and a power applying electrode disposed above the base member, housed in a vacuum chamber. The substrate serving as an electrode opposed to the power applying electrode in the vacuum chamber is guided into the vacuum chamber. The power applying electrode is electrically connected to a high frequency power source and the high frequency power source applies a power between the power applying electrode and the substrate. Then, a plasma is generated in a discharge space between the power applying electrode and the substrate to decompose the reactive gas introduced into the vacuum chamber and thus form a thin film as a deposited film on the substrate. In the ordinary plasma processing systems, the power applying electrode is fixed through an insulator to an internal wall of the vacuum chamber, for example, as disclosed in Japanese Patent Application Laid-Open No. 9-235676.
It was feasible heretofore to deposit thin films of amorphous semiconductors and the like on the substrate by use of the deposited film forming apparatus of this type. Since the amorphous semiconductors, e.g. amorphous silicon, can form a thin film in a small thickness and in a large area, provide a high degree of freedom for composition, and present controllability of electrical characteristics and optical characteristics within a wide range, they are recently drawing attention as materials for various semiconductor devices. Particularly, amorphous silicon is becoming a focus of attention as a material for solar cells, because it has such features that the absorption coefficient thereof is larger near the peak of the solar energy spectrum than those of crystalline silicon, that formation temperatures are low, and that a film can be directly formed from a source gas onto a substrate by glow discharge.
As for the solar cells increasingly valued as part of future new energy measures, reduction in cost and improvement in performance are significant subjects of research and development for the time being. Concerning the performance, solar cells with considerably high conversion efficiency have been yielded so far, but the reduction in cost of solar cells is not satisfactory yet. The reason is that the film forming rate of amorphous silicon is small.
A variety of proposals have been made heretofore as methods of forming a film of amorphous silicon at a high speed. An example is a method of decreasing the distance between the power applying electrode and the substrate, as disclosed in Japanese Patent Publication No. 5-56850.
During formation of a thin film of amorphous silicon or the like, the substrate, the power applying electrode, the discharge furnace, etc. are heated to a desired temperature in order to enhance the optical and electrical characteristics of the resultant thin film. Since electrons and ions accelerated by the plasma discharge collide with the substrate and power applying electrode, their temperatures increase. As a consequence, the substrate and power applying electrode undergo thermal expansion, which caused the substrate and power applying electrode to deform, e.g., warp, bend, or curve, as compared with their shapes set at room temperature.
The substrate undergoes no deformation or at most little deformation as long as the substrate is fixed to a substrate holder. The substrate holder is normally provided with a heater or the like and the substrate holder is considerably larger than the substrate. Therefore, the substrate holder is more resistant to deformation than the substrate. In the case of a belt-like substrate or the like guided into the discharge chamber in order to continuously convey the substrate in the roll-to-roll system without use of the substrate holder, the warpage and deformation of the substrate can be suppressed, for example, by securing the substrate by attraction of magnets at the edge portions of the substrate or by increasing the tensile force (tension) on the substrate. However, a thin film will inevitably be deposited on the power applying electrode, so that the power applying electrode tends to deform because of the stress of the thus deposited film. The conventional deposited film forming systems were not constructed in such structure as to hold the power applying electrode about to deform, and thus fine warpage or deformation was experienced.
Influence is little from the deformation of the power applying electrode where the distance between the power applying electrode and the substrate (electrode-substrate distance) is large. However, where the electrode-substrate distance is set small in order to increase the film forming rate, as disclosed in Japanese Patent Publication No. 5-56850, even fine deformation of the power applying electrode can cause a non-negligible range of influence on the electrode-substrate distance. As a result, the unevenness of the electrode-substrate distance resulted in nonuniformity of the plasma and partial difference of film forming rate, thereby posing a problem of unevenness of the film thickness. This problem would be a significant issue where the substrate is conveyed, particularly, where the film is formed while conveying the substrate in the roll-to-roll system.
Further, the deformation of the power applying electrode also makes the distance uneven between the power applying electrode and the base member supporting it, which can cause the plasma to intrude into the space (or gap) between them or which can induce an abnormal discharge in the space. It resulted in waste of the source gas, or generation of polysilane powder because of the undesired discharge state, thus degrading the maintainability of the apparatus and the mass producibility of thin films.
An object of the present invention is to provide a deposited film forming apparatus that can prevent intrusion of a plasma into a space between a base member and a power applying electrode due to deformation of the power applying electrode disposed above the base member in a vacuum chamber, and suppress occurrence of an abnormal discharge in the space.